1. Field of the Invention
The present invention relates to a structure of an electronic device. In particular, the present invention relates to an active matrix electronic device having a thin film transistor (TFT) formed on an insulating body.
2. Description of the Related Art
Flat panel displays have been drawing attention in recent years as substitutes for LCDs (liquid crystal displays), and research into such displays is proceeding apace.
LCDs can roughly be divided into two types of driving methods. One is a passive matrix type using an LCD such as an STN-LCD, and the other is an active matrix type using an LCD such as a TFT-LCD. EL displays can also be similarly broken down roughly into two types; one a passive type, and the other an active type.
For the passive type, wirings which become electrodes are arranged in portions above and below EL elements. Voltages are applied to the wirings in order, and the EL elements turn on due to the flow of an electric current. On the other hand, each pixel has a TFT with the active matrix type, and a signal can be stored within each pixel.
A schematic diagram of an active matrix EL display device is shown in FIGS. 19A and 19B. FIG. 19A is a schematic diagram of an entire circuit, and the circuit has a pixel portion 1853 in its center. Gate signal line driver circuits 1852 for controlling the gate signal lines are arranged to the left and right of the pixel portion. The arrangement may also bw on only one side, left or right, but it is preferable to use both positions as shown in FIG. 19A considering such reasons as operational efficiency and reliability. A source signal line driver circuit 1851 for controlling source signal lines is arranged above the pixel portion. A circuit for one pixel in the pixel in the pixel portion 1853 of FIG. 19A is shown in FIG. 19B. Reference numeral 1801 denotes a TFT which functions as a switching element during write in of a signal to the pixel (hereafter referred to as switching TFT) in FIG. 19B. Reference numeral 1802 denotes a TFT (hereafter referred to as EL driver TFT) which functions as an element (electric current control element) for controlling electric current supplied to EL elements 1803. From the fact that it is good for TFT operation to have a source region connected to ground, and from limitation on the fabrication of the EL elements 1803, p-channel TFTs are used as the EL driver TFTs. A general structure in which the EL driverTFT is arranged between an anode of the EL element 1803 and an electric current supply line 1807 is often employed. Reference numeral 1804 denotes a storage capacitor for storing a signal (voltage) input from a source signal line 1806. One terminal of the storage capacitor 1804 of FIG. 19B is connected to ther electric current supply line 1807, but a special-purpose wiring may also be used. A gate terminal of the switching TFT 1801 is connected to a gate signal line 1805, and a source terminal of the switching TFT 1801 is connected to the source signal line 1806. Further, a drain terminal of the EL driver TFT 1802 is connected to the anode or a cathode of the EL element 1803, and a source terminal of the EL driver TFT 1802 is connected to the electric current supply line 1807.
The EL elements have a layer (hereafter referred to as an EL layer) containing an organic compound in which electroluminescence (luminescence generated by application of an electric field) is obtained, the anode, and the cathode. There is emission of light in the organic compound when returning to a base state from a singlet excitation state (fluorescence), and when returning to a base state from a triplet excitation state (phosphorescence), and it is possible to apply both types of light emission with the present invention.
Note that all layers formed between the anode and the cathode are defined as EL layers throughout this specification. These layers include, specifically, layers such as a light emitting layer, a hole injecting layer, an electron injecting layer, a hole transporting layer, and an electron transporting layer. Specifically, EL element may have a structure in which an anode, a light emitting layer, and a cathode are laminated in order. In addition, structures such as one in which an anode, a hole injecting layer, a light emitting layer, and a cathode, and one in which an anode, a hole transporting layer, a light emitting layer, and an electron transporting layer are laminated in order, may also be used.
Further, elements formed by an anode, an EL layer, and a cathode are referred to as EL elements throughout this specification.
Circuit operation of the active matrix electronic device is explained next with reference to FIGS. 19A and 19B. First, the gate of the switching TFT 1801 opens when the gate signal line 1805 is selected, a voltage is applied to a gate electrode of the switching TFT 1801, and the switching TFT 1801 is placed in a conducting state. The signal (voltage) of the source signal line 1806 is thus stored in the storage capacitor 1804. The voltage of the storage capacitor 1804 becomes a voltage VGS between the gate and the source of the EL driver TFT 1802, and therefore the electric current, which responds to the storage capacitor 1804 voltage, flows in the EL driver TFT 1802 and in the EL element 1803. As a result, the EL element 1803 turns on.
The brightness of the EL element 1803, namely the amount of electric current flowing in the EL element 1803, can be controlled by VGS. VGS is the voltage stored in the storage capacitor 1804, and is the signal (voltage) to be input to the source signal line 1806. In other words, the brightness of the EL element 1803 is controlled by controlling the signal (voltage) to be input to the source signal line 1806. Finally, the gate signal line 1805 is unselected, the gate of the switching TFT 1801 closes, and the switching TFT 1801 is placed in a non-conducting state. The electric charge stored in the storage capacitor 1804 continues to be stored at this point. VGS is therefore stored as is, and the electric current in response to VGS continues to flow in the EL driver TFT 1802 and in the EL element 1803.
Information regarding the above explanation is reported upon in papers such as the following: xe2x80x9cCurrent Status and Future of Light-emitting Polymer Display Driven by Poly-Si TFTxe2x80x9d, SID99 Digest, p. 372; xe2x80x9cHigh Resolution Light Emitting Polymer Display Driven by Low Temperature Polysilicon Thin Film Transistor with Integrated Driverxe2x80x9d, ASIA DISPLAY 98, p. 217; and xe2x80x9c3.8 Green OLED with Low Temperature Poly-Si TFTxe2x80x9d, Euro Display 99 Late News, p. 27.
Along with high definition, large size screens are sought after with active matrix EL displays. However, the increase in the length of wirings which accompanies making larger screens becomes a cause of problems such as insufficient write in time and dispersion in the electric current supplied. In particular, the dispersion in the amount of electric current supplied to the EL element due to the resistance of the electric current supply lines is directly connected with display irregularities such as uneven brightness within the screen and crosstalk, and therefore is a burden to making screens larger.
A method of reducing the resistance in the electric current supply lines for each pixel by increasing the number of electric current supply lines can be given as a way of solving the above problem. However, simply increasing the number of wirings in the pixel portion or increasing the cross sectional area of the wirings invites a reduction in the aperture ratio, and cannot be said to be a desirable method.
A novel pixel structure not found conventionally is thus sought after in order to reduce the wiring resistance while maintaining a high aperture ratio.
The present invention answers such demands, and an object of the present invention is to provide an electronic device in which the number of electric current supply routes can be increased and in which wiring resistance can be reduced by using pixels having a novel structure.
The following means are discussed in order to resolve the problems with the above conventional techniques.
The present invention focuses on the fact that, in a structure of a pixel portion of an electronic device, gate signal lines and source signal lines existing in the pixel portion, in addition to electric current supply lines, are maintained at a certain fixed electric potential outside of a period for performing signal write in of signals to pixels. With the present invention, the electric potential of the gate signal lines and the source signal lines during a sustain (turn on) period which does not overlap with an address (write in) period is set equal to the electric potential of the electric current supply lines, and the gate signal lines or the source signal lines can be used as electric current supply lines by electrically connecting them to electric current supply lines through TFTs.
A method of adding new electric current supply lines is the simplest way of increasing the electric current supply paths. However, with the present invention the source signal lines and the gate signal lines can be utilized as routes of electric current supply, and therefore it becomes possible to increase the electric current supply paths with greater efficiency than by simply adding electric current supply lines as described above. As a result, the wiring resistance is lowered, and it becomes possible to eliminate such as uneven brightness and crosstalk. This can contribute greatly to an increase in quality.
Structures of the electronic device of the present invention are stated below.
According to a first aspect of the present invention, an electronic device comprising a source signal line side driver circuit, a gate signal line side driver circuit, and a pixel portion; is characterized in that:
the pixel portion has a plurality of source signal lines, a plurality of gate signal lines, a plurality of electric current supply lines, a connection control line, and a plurality of pixels;
each of the plurality of pixels has an EL element, and a first connection transistor or a second connection transistor;
a gate electrode of the first connection transistor is electrically connected to the connection control line;
one of a source region and a drain region of the first connection transistor is electrically connected to any one of the plurality of source signal lines, and the other one of the source region and the drain region is electrically connected to any one of the plurality of electric current supply lines;
a gate electrode of the second connection transistor is electrically connected to the connection control line; and
one of a source region and a drain region of the second connection transistor is electrically connected to any one of the plurality of gate signal lines, and the other one of the source region and the drain region is electrically connected to any one of the plurality of electric current supply lines.
According to a second aspect of the present invention, an electronic device comprising a source signal line side driver circuit, a gate signal line side driver circuit, and a pixel portion; is characterized in that:
the pixel portion has a plurality of source signal lines, a plurality of gate signal lines, a plurality of electric current supply lines, a connection control line, and a plurality of pixels;
each of the plurality of pixels has an EL element and a connection transistor;
a gate electrode of the connection transistor is electrically connected to the connection control line; and
one of a source region and a drain region of the connection transistor is electrically connected to any one of the plurality of source signal lines, and the other one of the source region and the drain region is electrically connected to any one of the plurality of electric current control lines.
According to a third aspect of the present invention, an electronic device, comprising a source signal line side driver circuit, a gate signal line side driver circuit, and a pixel portion; is characterized in that:
the pixel portion has a plurality of source signal lines, a plurality of gate signal lines, a plurality of electric current supply lines, a connection control line, and a plurality of pixels;
each of the plurality of pixels has an EL element and a connection transistor;
a gate electrode of the connection transistor is electrically connected to the connection control line; and
one of a source region and a drain region of the connection transistor is electrically connected to any one of the plurality of gate signal lines, and the other one of the source region and the drain region is electrically connected to any one of the plurality of electric current control lines.
According to a fourth aspect of the present invention, an electronic device comprising a source signal line side driver circuit, a gate signal line side driver circuit, and a pixel portion; is characterized in that:
the pixel portion has a plurality of source signal lines, a plurality of gate signal lines, a plurality of electric current supply lines, a connection control line, and a plurality of pixels;
each of the plurality of pixels has an EL element, a first connection transistor, and a second connection transistor;
a gate electrode of the first connection transistor is electrically connected to the connection control line;
one of a source region and a drain region of the first connection transistor is electrically connected to any one of the plurality of source signal lines, and the other one of the source region and the drain region is electrically connected to any one of the plurality of electric current supply lines;
a gate electrode of the second connection transistor is electrically connected to the connection control line; and
one of a source region and a drain region of the second connection transistor is electrically connected to any one of the plurality of gate signal lines, and the other one of the source region and the drain region is electrically connected to any one of the plurality of electric current supply lines.
According to a fifth aspect of the present invention, an electronic device comprising a source signal line side driver circuit, a gate signal line side driver circuit, and a pixel portion; is characterized in that:
the pixel portion has a plurality of source signal lines, a plurality of gate signal lines, a plurality of electric current supply lines, a connection control line, and a plurality of pixels;
each of the plurality of pixels has an EL element, a first connection transistor, and a second connection transistor;
a gate electrode of the first connection transistor is electrically connected to the connection control line;
one of a source region and a drain region of the first connection transistor is electrically connected to any one of the plurality of source signal lines, and the other one of the source region and the drain region is electrically connected to any one of the plurality of electric current supply lines;
a gate electrode of the second connection transistor is electrically connected to the connection control line; and
one of a source region and a drain region of the second connection transistor is electrically connected to any one of the plurality of source signal lines, and the other one of the source region and the drain region is electrically connected to any one of the plurality of gate signal lines.
According to a sixth aspect of the present invention, an electronic device comprising a source signal line side driver circuit, a gate signal line side driver circuit, and a pixel portion; is characterized in that:
the pixel portion has a plurality of source signal lines, a plurality of gate signal lines, a plurality of electric current supply lines, a connection control line, and a plurality of pixels;
each of the plurality of pixels has an EL element, a first connection transistor, and a second connection transistor;
a gate electrode of the first connection transistor is electrically connected to the connection control line;
one of a source region and a drain region of the first connection transistor is electrically connected to any one of the plurality of gate signal lines, and the other one of the source region and the drain region is electrically connected to any one of the plurality of electric current supply lines;
a gate electrode of the second connection transistor is electrically connected to the connection control line; and
one of a source region and a drain region of the second connection transistor is electrically connected to any one of the plurality of source signal lines, and the other one of the source region and the drain region is electrically connected to any one of the plurality of gate signal lines.
According to a seventh aspect of the present invention, an electronic device comprising a source signal line side driver circuit, a gate signal line side driver circuit, and a pixel portion; is characterized in that:
the pixel portion has a plurality of source signal lines, a plurality of gate signal lines, a plurality of electric current supply lines, a connection control line, and a plurality of pixels;
each of the plurality of pixels has an EL element, a first connection transistor, a second connection transistor, and a third connection transistor;
a gate electrode of the first connection transistor is electrically connected to the connection control line;
one of a source region and a drain region of the first connection transistor is electrically connected to any one of the plurality of source signal lines, and the other one of the source region and the drain region is electrically connected to any one of the plurality of electric current supply lines;
a gate electrode of the second connection transistor is electrically connected to the connection control line;
one of a source region and a drain region of the second connection transistor is electrically connected to any one of the plurality of gate signal lines, and the other one of the source region and the drain region is electrically connected to any one of the plurality of electric current supply lines;
a gate electrode of the third connection transistor is electrically connected to the connection control line; and
one of a source region and a drain region of the third connection transistor is electrically connected to any one of the plurality of source signal lines, and the other one of the source region and the drain region is electrically connected to any one of the plurality of gate signal lines.
According to an eighth aspect of the present invention, an electronic device, according to any of the first to seventh aspects of this invention, is characterized in that:
the polarity of an EL driver transistor is p-channel type when one of a source region and a drain region of the EL driver transistor is electrically connected to an anode of the EL element; and
the polarity of an EL driver transistor is n-channel type when one of the source region and the drain region of the EL driver transistor is electrically connected to a cathode of the EL element.
According to a ninth aspect of the present invention, an electronic device, according to any of the first to eighth aspects of this invention, is characterized in that:
the polarity of a switching transistor is identical to the polarity of the EL driver transistor.
According to a tenth aspect of the present invention, an electronic device, according to any of the first to ninth aspects of this invention, is characterized in that:
the gate signal lines are composed of aluminum, or of a material having aluminum as its main constituent.
According to an eleventh aspect of the present invention, a method of driving an electronic device, comprising n-bit gray scale control by controlling the length of time during which an EL element is turned on, is characterized in that:
a frame period has n subframe periods SF1, SF2, . . . , Sfn;
the n subframe periods have address (write in) periods Ta1, Ta2, . . . , Tan, and sustain (turn on) periods Ts1, Ts2, . . . , Tsn, respectively;
the length of the sustain (turn on) periods is set so that Ts1::Ts2:: . . . ::Tsn=2(nxe2x88x921)::2(nxe2x88x922):: . . . ::20;
a first connection transistor or a second connection transistor is conductive during the sustain (turn on) periods which do not overlap with the address (write in) periods; and
an electric current supply line and a source signal line electrically connected to a source region and a drain region of the first connection transistor are placed in a conducting state, or an electric current supply line and a source signal line electrically connected to a source region and a drain region of the second connection transistor are placed in a conducting state.
According to a twelfth aspect of the present invention, a method of driving an electronic device, comprising n-bit gray scale control by controlling the length of time during which an EL element is turned on, is characterized in that:
a frame period has n subframe periods SF1, SF2, . . . , Sfn;
the n subframe periods have address (write in) periods Ta1, Ta2, . . . , Tan, and sustain (turn on) periods Ts1, Ts2, . . . , Tsn, respectively;
the length of the sustain (turn on) periods is set so that Ts1::Ts2:: . . . ::Tsn=2(nxe2x88x921)::2(nxe2x88x922):: . . . ::20; and
the supply of electric current to the EL element during the sustain (turn on) periods which do not overlap with the address (write in) periods is performed by an electric current supply line and: a source signal line electrically connected to the electric current supply line through a first connection transistor; or a gate signal line electrically connected to the electric current supply line through a second connection transistor.
According to a thirteenth aspect of the present invention, a method of driving an electronic device, comprising n-bit gray scale control by controlling the length of time during which an EL element is turned on, is characterized in that:
a frame period has n subframe periods SF1, SF2, . . . , Sfn; the n subframe periods have address (write in) periods Ta1, Ta2, . . . , Tan, and sustain (turn on) periods Ts1, Ts2, . . . , Tsn, respectively;
the length of the sustain (turn on) periods is set so that Ts1::Ts2:: . . . ::Tsn=2(nxe2x88x921)::2(nxe2x88x922):: . . . ::20; and
an electric current supply line and a number i row gate signal line electrically connected to a source region and a drain region of a connection transistor are placed in a conducting state with the number i gate signal line is unselected.